1. Field of the Invention
The present invention relates to a light scanning optical system, such as a laser printer or a laser display, which displays an image by scanning light, and to an image forming apparatus that uses the scanning optical system. In particular, the present invention relates to a scanning optical system that optically increases a light scanning angle using a micro electro mechanical system (hereinafter referred to as the “MEMS”) produced through a semiconductor process.
2. Related Background Art
Heretofore, in an image forming apparatus of a light scanning type such as a laser printer or a laser display, in order to meet demand for a large deflection angle and high-speed scanning, a so-called polygon mirror has been used which is a mirror that is capable of rotating a rotation body having multiple reflection surfaces at high speed. Also, a distance to a scan target point can be reduced by increasing a beam deflection angle, so that various methods for optically increasing the light deflection angle have been proposed.
As an example of the methods, Japanese Patent Application Laid-Open No. H06-003616 discloses a polygon scanner in which an optical system is added to a polygon mirror, thereby increasing the deflection angle. FIGS. 1 and 2 each show a construction of the conventional polygon scanner. With this construction, a light beam entering the polygon mirror is first brought to pass through an optical system and then is caused to enter a reflection surface of the same polygon mirror again, thereby making it possible to increase the deflection angle as compared with a case where deflection is performed only once.
By the way, in recent years, a technique called a micro electro mechanical system (MEMS) produced with a semiconductor process technique has received attention. In particular, a light scanner that uses a light scanning means based on the MEMS technique has received special attention. As an example of the light scanner, U.S. Pat. No. 5,606,447 discloses a light scanning device having a construction shown in FIG. 3. In this drawing, a planar movable plate and a torsion bar axially supporting the movable plate in a rockable manner are formed integrally with a silicon substrate. Also, a planar coil that generates a magnetic field through energization is provided in an upper surface edge portion of the movable plate, a total reflecting mirror is provided in an upper surface center portion surrounded by the planar coil, and permanent magnets that exert magnetic fields on the planar coil are arranged. With this construction, when current is applied to the coil, a Lorentz force is generated and the mirror is driven. By driving the mirror in the vicinity of a mechanical resonance frequency, a large deflection angle is obtained. Such a resonance-type light scanning means is small in size and is light in weight, so that various features are attained such as high-speed driving and small driving energy. However, the magnitude of the deflection angle depends on the torsion angle of the torsion bar, so that in order to obtain a large deflection angle, the torsion bar portion needs to be increased in length, which leads to an increase in device size. Also, when a beam deflection surface is increased in size, it becomes difficult to maintain a high resonance frequency, which means that it is generally difficult to increase the size of the reflection surface as well as the deflection angle.
Methods for increasing the deflection angle of such an MEMS device are also proposed. As an example of the methods, Japanese Patent Application Laid-Open No. H11-259888 discloses a deflection apparatus and an optical information recording and reproducing head having a construction shown in FIG. 4. Generally, as shown in FIG. 5, if a mechanical deflection angle is referred to as “θ”, a light deflection angle becomes 2θ. In the deflection apparatus having the construction shown in FIG. 4, however, a galvano-mirror, whose reflection surface is constructed so as to be rotatable in a predetermined direction, is provided and a fixed mirror is arranged so as to oppose the reflection surface of the galvano-mirror. With this construction, a light flux entering the galvano-mirror is caused to enter the fixed mirror at least once and a light flux reflected by the fixed mirror is reflected by the galvano-mirror again, thereby widening the deflection angle. As a result, as shown in FIG. 4, a deflection angle of 4θ is obtained.
In Japanese Patent Application Laid-Open No. H06-003616, however, the polygon mirror is used as a light deflector, and the rotation center of the polygon mirror and a light deflection point (reflection point) do not coincide with each other. Consequently, the light deflection point is displaced in accordance with rotation of the polygon mirror, which causes lowering of optical performance. For instance, points that should be in a mutually conjugate relationship become non-conjugate. Also, optical path interference occurs between incident light and deflected light.
Also, with the construction disclosed in Japanese Patent Application Laid-Open No. H11-259888, although a deflection angle that is four times as large as the mechanical deflection angle θ is obtained indeed, a first deflection point and a second deflection point are spaced apart from each other in an in-plane direction that is perpendicular to a rotation axis, so that the size of the reflection surface is unavoidably increased in a direction that is perpendicular to the torsion bar (in the in-plane direction of the paper plane). Also, when the distance between the fixed mirror and the movable mirror is reduced, optical path interference occurs, so that it becomes impossible to substantially increase an angle of deflection.